In English though, "to tan" is also usually intransitive here, unless you consider your skin to be understood as the object. I see that French allows either in this situation. In English, there would be a lot of emphasis if I said "I am tanning myself." (as opposed to someone else?? how would you do that? So, we usually just say "I am tanning." I suppose it could be possible to help someone else tan if you worked in a tanning salon.)